1. Field of the Invention
The present invention relates to an optical film having a high contrast ratio over a wide range and capable of suppressing the color shift, a production method of the optical film, and an optically-compensatory film, a polarizing plate and a liquid crystal display device each using the optical film.
2. Description of the Related Art
A liquid crystal display device is widely used for a monitor of personal computers or potable devices or for a television because of its various advantages such as low voltage/low power consumption and capability of size and thickness reduction. For such a liquid crystal display device, various modes are proposed according to the aligned state of liquid crystal molecules in the liquid crystal cell. Conventionally, a TN mode having such an aligned state as that liquid crystal molecules are twisted at about 90° toward the upper substrate from the lower substrate of the liquid crystal cell is predominating.
In general, a liquid crystal display device comprises a liquid crystal cell, an optically-compensatory sheet and a polarizer. The optically-compensatory sheet is used for canceling the image coloration or enlarging the viewing angle, and a film obtained by coating a liquid crystal on a stretched birefringent film or transparent film is used therefor.
However, the requirement with respect to the viewing angle dependency of a liquid crystal display device for televisions having a large screen and envisaging viewing from various angles is severe. To meet the requirement for more enhancement of the viewing angle characteristics, studies are being made on a liquid crystal display device different from the TN mode, such as IPS (in-plane switching) mode, OCB (optically compensatory bend) mode and VA (vertically aligned) mode. Particularly, the IPS mode ensures high contrast, good viewing angle characteristics and relatively high production yield and therefore, this mode is attracting attention as a liquid crystal display device for TV.
In the IPS mode, slight light leakage in the diagonal oblique incident direction at the black display time becomes a problem as a cause of reduction in the display quality. As one means for improving the color tone or viewing angle at black display, a technique of disposing an optically-compensatory material having birefringent characteristics between the liquid crystal layer and the polarizing plate is being studied also in the IPS mode. For example, there is disclosed an invention where light leakage based on the deviation of the crossing angle of polarization axes from the right angle when the crossed polarizing plates are viewed from the oblique direction is suppressed by using a phase difference film having Nz of 0.4 to 0.6 and an in-plane phase difference of 200 to 350 nm (see, JP-A-2004-4642 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”))